MATLAB: Problem with Interp1

Question

Hi, I cannot get interp1 to give me the correct answer. I have a simple, data set (see below) and all I want to do if use interpolation to give me the x value at y=thresh

thresh=225.5

My code is:

   vq1 = interp1(x,y,thresh,'pchip')

This results in vq1=12207908.76

x=

          1.00
          2.00
          3.00
          4.00
          5.00
          6.00
          7.00
          8.00
          9.00
         10.00
         11.00
         12.00
         13.00
         14.00
         15.00
         16.00
         17.00
         18.00
         19.00
         20.00
         21.00
         22.00
         23.00
         24.00
         25.00
         26.00
         27.00
         28.00
         29.00
         30.00
         31.00
         32.00
         33.00
         34.00
         35.00
         36.00
         37.00
         38.00
         39.00
         40.00
         41.00
         42.00

y=

          430.00
          422.00
          445.00
          438.00
          423.00
          435.00
          432.00
          422.00
          419.00
          440.00
          403.00
          427.00
          444.00
          437.00
          429.00
          417.00
          440.00
          441.00
          432.00
          425.00
          413.00
          370.00
          271.00
          139.00
           63.00
           28.00
           36.00
           27.00
           25.00
           27.00
           15.00
           27.00
           22.00
           29.00
           20.00
           19.00
           27.00
           17.00
           17.00
           15.00
           22.00
           17.00

Answer 1

If you want to find the ‘x’-values where ‘y’ equals ‘thresh’, you need to reverse the order of ‘x’ and ‘y’ in the interp1 call. However with your data, you first have to find the approximate index of that value, since you cannot simply reverse the order of the entire vector.

Try this:

yval = find(y >= thresh, 1, 'last');                                % Define Region To Interpolate
vq1 = interp1(y(yval-1:yval+1), x(yval-1:yval+1), thresh,'pchip')   % Interpolate In Region
vq1 =
    23.3818